There is a known control system for a forward driven drilling apparatus, wherein the control of the driving force is dependent on the torque of the drilling apparatus. The tool in the drill head cuts the tunnel front wall through rotation. The rotary motion is transmitted to the drill head from the tunnel opening, in general, by the rotating soil-conveying tubes. The torque of the conveying tube system is monitored and, as the torque drops, the drilling apparatus is driven forward.
An applicable method is also to provide the tool in the drill head with a relatively short feed of its own, the length of which the tool moves on when drilling the tunnel. When the feed travel comes to its end, the tool reverts to start and the entire drilling apparatus is driven forward respectively. This procedure is known, for example, from the GB-publication No. 2 091 316 and the U.S. Pat. No. 4,167,289.
The disadvantage of a torque monitoring control method is the growth of torque when the tunnel drive advances. The farther the tunnel work advances the more the resistance of rotation drops, which can be caused for instance by the conveying tubes transmitting the torque. Therefore, when the tunnel drive advances, the control system becomes insensible to tool hindrances far from the tunnel opening and damage to the tool is quite possible.
In a system where the tool is provided with an individual built-in feed motion in the drill head, the mounting of the tool becomes complicated and even the control system must be of a kind that senses the qualities and variations of soil ahead.